As shown in FIG. 1, a conventional microwave oven includes a heating chamber 1 for cooking food, a turntable 2 positioned in the chamber for supporting the food, a turntable motor 3 for turning the turntable, a key application part 9 having keys for a user to select cooking functions, a microcomputer 4 for controlling the microwave oven according to the key signals applied from the key application part 9, a magnetron 6 for generating microwaves, a power supply part 5 for supplying power to the magnetron 6 to operate the magnetron 6 under control of the microcomputer 4, and a waveguide 8 for transmitting the microwaves generated in the magnetron 6 to the heating chamber 1 through an opening 7 formed in the wall of the heating chamber 1.
Operation of the foregoing conventional microwave oven is as follows.
First, when a user puts food on the turntable 2 in the heating chamber 1 and presses keys selected to carry out a desired cooking function through the key application part 9 for cooking, the microcomputer 4 controls the turntable motor 3 to turn the turntable 2 and, at the same time, controls the power supply part 5 to supply power to the magnetron 6.
Upon supplying power to the magnetron 6, microwaves are generated and transmitted to the heating chamber 1 through the opening 7 and the waveguide 8 to cook the food.
However, the conventional microwave oven generates microwaves at different outputs and with different efficiencies depending on the load of food in the oven.
For example, as shown in FIG. 2, if a microwave oven is designed to have an output of 700 W and an efficiency of 50% for 2000 cc of food, the output of the microwave oven will fall to 660 W and the efficiency will drop to 46% for a food load of 1000 cc, and the output of the microwave oven will fall to 610 W and the efficiency will drop to 38% for a food load of 500 cc. Therefore, there has been a problem that an optimum cooking condition can not be obtained when the food load in the oven varies.